Depression is a complex, severe debilitating mental disorder that affects about 10% of Americans. While it is well established that environmental factors, such as stress, plays an etiology role, the brain mechanisms, particularly the role of specific neural circuits mediating the pathogenesis of depression, remain to be elucidated. Chronic social defeat stress (cSDS) in mice is a highly relevant, validated model to study brain mechanisms of depression. This behavior paradigm has been shown to induce morphological and functional changes in multiple brain regions including the prefrontal cortex (PFC), which is interconnected with other limbic brain regions such as the nucleus accumbens (NAc), ventral tegmental area, hippocampus and amygdala. It is not clear whether a particular neuronal type in the PFC, defined by its synaptic connectivity, is more relevant to the pathogenesis of depression. Using a recently developed neuronal activity reporter mouse line, termed fosTRAP, the applicant's lab has determined that acute and chronic social defeat stress activate distinct populations of projection neurons in the PFC. This raises the question of whether specific circuit connectivity is pertinent to depression and if future therapeutic strategies could be devised, using a `precision medicine in psychiatry' approach, to target the relevant circuits to combat the core symptoms of depression while alleviating off target effects. In this R21 proposal, the expertise of two junior faculty laboratories (Qiu, neurophysiology and functional circuit mapping; Ferguson, mouse depression behavior and optogenetics), will be merged to test the hypothesis that chronic social defeat-induced, depression-related behaviors are encoded within a specific neural circuit in the PFC. These PIs will employ the fosTRAP:AI14 reporter mouse combined with tamoxifen to gain genetic access to the prefrontal neurons that are activated by cSDS. They will further explore whether disrupted synaptic homeostasis selectively occurs in the proportion of L5 neurons that are activated by the chronic social defeat stress. This will be investigated using whole cell patch clamp electrophysiology and laser scanning photostimulation for functional circuit mapping studies in NAc-projecting L5 prefrontal neurons in fosTRP:AI14 reporter mice following cSDS (Aim 1). They will also test whether disrupted synaptic homeostasis occurs selectively in the susceptible mice populations. In Aim 2, they will use targeted optogenetic manipulation of neural activity in the L5 PFC projection neurons that are selectively activated by cSDS and also selectively express opsins. These investigators will test the novel hypothesis that optogenetic inhibition of this specific neuron ensemble during the acquisition of depressive-like behavior confers resistance, while repeated activation of these neurons leads to susceptibility. This study could reveal a paradigm-shifting practice in circuit-based therapeutics aimed at restoring prefrontal synaptic homeostasis and could establish a specific corticolimbic circuit as a lead target for preventing the development of depression, which is otherwise not possible by previous studies examining an indiscriminate population of PFC neurons.